The mechanical erection of a tray from a blank of the kind described above has hitherto presented difficulties because each edge part, which is guided during erection by the inward movement of a mechanical finger into the slit of the respective tongue, has the tendency to bend together with the respective tongue about a line somewhat coaxial with the pivot line, or root of the tongue.
Due to this, it is no longer possible to form a double bend from an outer edge part surrounding the tongue, and to insert a wedge-shaped portion thereof into the tongue slit.